Energy modeling programs, such as RETScreen (free; shown below), T-Sol ($1,000+), or F-Chart ($400), allow simulation of solar heating system performance before any equipment is purchased. You enter information such as the collector model, location, size of load, months the system will be used, collector tilt and azimuth, amount of proposed storage, efficiency of heat exchanger, and so on. Then you can change factors to see what will work best. The program report will give you a very good idea on how your system will perform in the real world because it uses actual weather data and actual collector performance in the calculations.